1. Field of the Invention
The present invention relates to a synchronizing detector circuit. More particularly, it relates to an ACC (automatic color signal gain control) detector circuit or a killer detector circuit in a color television circuit.
2. Description of the Prior Art
In order to reproduce stable color pictures irrespective of the field intensity of the color broadcast and to stably discriminate between a color broadcast and a black-and-white broadcast, it is desired in an ACC detector circuit or a killer detector circuit that the relationship between the amplitude voltage of a color burst signal and the ACC voltage or the killer voltage have good linearity. It is also desired that a change in the ACC voltage or the killer voltage caused by the change of the color burst signal be large. Further, since, at present, the above type of circuit has generally been constructed of a semiconductor integrated circuit, it is desirable that the number of detecting RC filters of the above circuit be small because the semiconductor integrated circuit should preferably have a small number of externally mounted terminals and components.
For an ACC detector circuit or a killer detector circuit, a synchronizing detector circuit has been developed by the applicants of the present application and is described in Japanese Patent Application No. 122509/1972 entitled "Synchronizing Detector Circuit".
This synchronizing detector circuit is shown in FIG. 3. In series with a constant current transistor Q.sub.1 which is controlled by a synchronizing pulse is a symmetrical differential circuit for effecting full-wave rectification. A constant current transistor circuits is connected in series with the differential circuit. In the symmetrical differential circuit, transistors Q.sub.4, Q.sub.5 and transistors Q.sub.6, Q.sub.7, which are respectively connected in differential form, are connected to the respective collectors of transistors Q.sub.2 and Q.sub.3. The connection between the differential circuit and the constant current circuit is made such that the collectors of transistors Q.sub.4 and Q.sub.6 of the different sets of differential transistors are connected to the constant current transistor Q.sub.8, while the collectors of the transistors Q.sub.5 and Q.sub.7 are connected to the transistor Q.sub.9. An output transistor Q.sub.10 is connected to the circuitry of the above arrangement in such a manner that its base is connected to the juncture between the constant current transistor Q.sub.9 and the differential transistors Q.sub.5 and Q.sub. 7, while its emitter is connected to the juncture between the other transistor Q.sub.8 of the constant current circuit and the differential transistors Q.sub.4 and Q.sub.6. The collector of the output transistor Q.sub.10 is connected to a filter circuit composed of a resistor R and a capacitor C.
In this circuit, the portion enclosed by broken lines in the figure is formed as a semiconductor integrated circuit. Thus, an externally mounted terminal 1 is applied with a supply voltage V.sub.CC, terminal 2 is connected to the filter circuit, terminal 3 is a synchronizing pulse input terminal, terminal 4 is supplied with a chroma signal having a color burst signal, and terminal 5 is an input terminal for a subcarrier signal.
The circuit arrangement also includes bias resistances of the transistors and resistances inserted for stably operating the transistors. Since they are self-explanatory to those skilled in the art, an explanation thereof is omitted.
In operation, a synchronizing pulse is derived from a horizontal synchronizing signal through, for example, a delay circuit, so as to be synchronized with the color burst signal. In the circuit, therefore, only when the synchronizing pulse is present, does the transistor Q.sub.1 operate, and a current based on the constant current characteristic of the transistor Q.sub.1 flows. Accordingly, the chroma signal at that time corresponds only to the color burst signal portion. Then, the differential circuit receives the two inputs of the color burst signal and the subcarrier signal in phase with and having the same frequency as the color burst signal, and provides an output proportional to the color burst signal. In addition, the color burst signal is compared in phase with the subcarrier signal and is distinguished from a noise signal.
Such an operation is carried out as a so-called full-wave rectification of the color burst signal as stated below. During the positive half cycle of the color burst signal, the transistor Q.sub.3 becomes conductive and the transistor Q.sub.2 becomes nonconductive, so that an output from the transistors Q.sub.6 and Q.sub.7 is obtained. On the other hand, during the negative half cycle of the color burst signal, the state is the converse to the above, and the transistor Q.sub.2 becomes conductive and the transistor Q.sub.3 becomes nonconductive, so that an output from the transistors Q.sub.4 and Q.sub.5 is obtained.
Since, the circuit employs constant current circuit transistors Q.sub.8 and Q.sub.9 as the load circuit of the synchronizing detector circuit and the base-emitter circuit of the output transistor Q.sub.10 is connected to the constant current circuit, the gain of the ACC voltage (also the killer voltage) can be made high relative to the burst input signal, and a sufficiently great ACC voltage (killer voltage) can be produced even from the single RC filter circuit.
Moreover, the base input of the output transistor Q.sub.10 is received from the juncture between the constant current transistor and the differential transistors, while the emitter input is received from the other similar juncture constituting a pair with the above-mentioned juncture. Therefore, the output voltage of the transistor Q.sub.10 supplied through its collector to the load filter circuit is as explained below.
When the alternating amplitude of the color burst signal V.sub.BUR is 0 volts, equal currents flow through the differential circuits, and the input current of the output transistor Q.sub.10 becomes zero, so that the output V.sub.out becomes 0 volts. When the color burst signal V.sub.BUR is sufficiently large, the maximum value of the output voltage V.sub.out becomes V.sub.CC -V.sub.BE8 -V.sub.CE10(sat), and is substantially equal to the supply voltage V.sub.CC.
Consequently, according to the above circuit, the ACC voltage or the killer voltage varies linearly approximately over a range of 0 volts - V.sub.CC volts depending on the variation of the alternating amplitude of the color burst signal as illustrated at l.sub.1 in FIG. 2, and outputs having large amounts of change can be produced. As a result, the ACC circuit or the killer circuit which is driven by this circuit is operated efficiently.
As the externally mounted component, one RC filter circuit suffices. Therefore, where the above circuit is constructed as a semiconductor integrated circuit, the numbers of the externally mounted terminals and components can be made small. With this proposed circuit, the various effects described above should be expected.
However, when the circuit was actually fabricated and operated, it was revealed that, as shown by a broken line l.sub.2 in FIG. 2, a so-called offset voltage V.sub.os arises, particularly during the reception of a weak field, because the ACC voltage or the killer voltage does not decrease with the lowering of the color burst signal. Needless to say, however, the output of the proposed circuit is satisfactory in comparison with the output of the above referred to synchronizing detector circuit (not provided with the output transistor Q.sub.10) as shown by a one-dot chain line l.sub.3 in the figure. (Refer to Japanese Patent Application No. 122509/1972).
The cause of the offset voltage V.sub.os was studied, and the following has been discovered.
In the previous explanation of operation, noise signals were assumed to be perfectly eliminated by synchronous detection. In actuality, however, some noise signals asynchronous with the phase of the color subcarrier of 3.58 MHz cannot be eliminated. On the other hand, the output is obtained from the output transistor Q.sub.10. In the circuit of FIG. 3, noise components can flow only in the direction of the arrows by the rectifying function of the base-emitter junction of the output transistor 210. Therefore, the noise component cannot flow in a direction opposite to the arrows. Accordingly, only the positive components of the noise (negative components in some circuit arrangements) are fed via the output transistor Q.sub.10 to the R-C filter circuit, to give rise to the offset voltage V.sub.os.